This invention relates to a composite materials formed from polymerizable surfactants and hydrophobic polymers. The invention relates in particular to nanoporous composites.
Liquid crystals (LC) are a class of molecules that posses some form of anisotropy. This anisotropy can be achieved in two ways: either the LC molecules have one molecular axis that is very different from the other axes of the molecule, or the LC molecules possess portions with very different solubilities. Lyotropic liquid crystals have portions with very different solubilities (i.e. surfactants). Lyotropic liquid crystals (LLC) are a subset of liquid crystals which are materials that exhibit properties between those of liquids and crystals. There are various kinds of liquid crystals with varying degrees of order. Some LCs exhibit only orientational order, other LCs exhibit both orientational order and positional order in one or two dimensions. The composite materials of the present invention contain lyotropic liquid crystals in which portions of the LLC have very different solubilities. Typically, but not always, a solvent is required to induce the liquid crystal order.
More specifically, LLC mesogens are amphiphillic molecules containing one or more hydrophobic organic tails and a hydrophilic headgroup (Collins, P. J, 1990). The amphiphillic character of these molecules encourages them to self-organize into aggregate structures, often in the presence of polar solvents, such as water, with the tails forming hydrophobic regions and the hydrophilic headgroups defining the interface of phase-separated polar or aqueous solvent domains. These LLC assemblies can be relatively simple individual structures, such as micelles and vesicles (as illustrated in FIG. 1 of Gin et al., 2001, top row) which are termed LLC aggregates herein or highly ordered, yet fluid, condensed LLC assemblies with specific nanometer-scale geometries, known collectively as LLC phases (as illustrated in FIG. 1 of Gin et al., 2001, bottom row) (Collins, 1990; Tiddy, 1980.) LLC assemblies can include both LLC phases and LLC aggregates.
Biological materials, such as bone and shell, exhibit unusual properties that have been associated with nanometer-scale structural features present in these materials. Considerable effort has been expended in attempts to develop method that allow the introduction of nanoscale structures into synthetic composite materials with a view to producing new materials with unusual properties. It has not been possible, in general, to employ conventional materials processing to readily achieve nanometer-scale structural control in the production of synthetic materials. Thus, the development of methods for constructing synthetic composites with nanometer scale organization has become the focus of significant levels of research in materials science. Covalent stabilization, e.g., through polymerization, of two and three-dimensional structures of self-assembled molecules has been applied to form materials with desired nanoscale structure (Clark et al., 1999.) U.S. Pat. No. 6,264,741, for example, relates to a method of making nanocomposites using polymerization of self-assembled components.
LLC assemblies can provide the geometry control needed for production of nanostructured materials (Gin et al. Syn Lett. 1999; and Miller et al., 1999.) The architectures of LLC aggregates and phases can incorporate hydrophobic and hydrophilic compounds in separate domains with well-defined nano-scale geometries. The amphiphillic self-assembly process localizes the headgroups of the LLC mesogens exclusively at the hydrophobic/hydrophilic interface (headgroups toward the hydrophilic phase).
The organization of LLC assemblies (both phases and aggregates) are primarily dependant on the concentrations of the LLC surfactant and polar or aqueous solvent, however, like thermotropic liquid crystals, the ordered structures are also a function of temperature. Heating a material having a LLC phase or aggregate above the clearing point of the LLC will form a homogeneous, isotropic melt. Cooling the isotropic LLC material below the clearing point will cause the LLC phase to reform. When an external force is exerted on an LLC phase, the material will flow and the shear can affect the phase orientation and crystal domain size. Adding (or removing) solvent can cause the phase to change, and adding enough solvent to dilute the surfactant to below the critical micellar concentration results in an isotropic solution.
LLC assemblies can be employed as templates for materials preparation. U.S. Pat. No. 6,054,111, for example, reports the use of LLC phases of amphiphillic block copolymers as templates for the preparation of mesoporous inorganic solids or metals by polymerization or other chemical reactions of precursors combined with the LLC copolymers (mesoporous is defined therein as having a pore diameter ranging from 2 to 60 nm). Preparation of the mesoporous solid is completed by removal of the LLC phase template.
While LLC assemblies can provide nanoscale structure, the assemblies are inherently fluid and often lack the robustness required for most materials applications. Jung et al., 2001 reports on the polymerization of monomers inserted into the hydrophobic portion of LLC phases of dioctadecyldimethylammonium bromide. In general discussion of the use of LLC (and other LC) phases as templates for materials preparation, the authors state that “complex phase changes” during polymerization “can frustrate the concept of direct ‘templating’.” The reference reports the results of styrene polymerization in cubic LLC phases. After polymerization the phase structure is maintained only if the system is kept at constant temperature. The authors report that as a result of phase separation the polymerization does not provide a stable copy of the LCC template phase.
The use of polymerizable or crosslinkable LLC mesogens allow LLC assemblies to be more generally employed as templates for materials having nanoscale structures.
Polymerizable or crosslinkable LLC mesogens are reactive amphiphiles which aggregate into the same type of LLC assemblies as their non-polymerizable analogues, but which can be covalently linked to their nearest neighbors in situ to form robust polymer networks that retain the original LLC structure even when the temperature is raised past the clearing point of the unpolymerized system or when excess solvents are added (Gin, D. L et al. Acc. Chem. Res.,2001; U.S. Pat. No. 5,849,215).
Micelles (Paleos, 1992), inverse micelles (Barton, 1996) and microemulsions (Capek, 1999) can be polymerized with retention of phase microstructure. U.S. Pat. No. 5,238,992 reports the preparation of polymer blends of controlled porosity by polymerization of monomers in micoremulsions. Larnellar assemblies such as vesicles (Ringsdorf et al., 1998), lipid microtubules (Schnur et al., 1987) and the lamellar (L) phase (Ringsdorf et al., 1998) have also been successfully polymerized. The polymerization of more geometrically complex, non-lamellar LLC phases has also been achieved. Several complex phases including the normal hexagonal phase (HI), and the inverted hexagonal phase (HII) (Gin et al., Acc. Chem. Res. 2001; Thundathil et al, 1980; Shibasaki et al., 1992; McGrath, 1996; Smith, et al.1997; and Srisiri et al., 1997) have been polymerized with retention of phase structure. Polymerization of the most geometrically complex, bicontinuous cubic (QII, Pn3m) phase was reported by Lee, Y. S. et al., 1995.) Resel et al. 2000 report a structural study of polymerized LLC phases of certain LLC salts. It is reported that the structural properties of polymerized films can be affected by the length of the hydrophobic tail of the LLC and the nature of the metal counterion used.
Polymerizable surfactants are molecules having at least a pair of hydrophobic and hydrophilic components and at least one polymerizable group, such as a vinyl group, in their structure (Karas(ed.), 1999.) Polymerizable surfactants have been employed in several technologies including polymer emulsions, particulate encapsulation, etc. As noted above, polymerizable surfactants can be used to form LLC phases in the production of useful materials with highly regular nano-scale architectural features (i.e. pores, etc.). Polymerizable groups such as acrylate, methacrylate or diene groups have often been employed because groups can be easily photopolymerized. Photopolymerization is the preferred polymerization scheme because it can be initiated “on command” by irradiation of light over a wide temperature range. Using photopolymerization methods, the sample can be heated or cooled (if necessary) to a temperature where the desired LLC phase exists and then photopolymerized by exposure to ultraviolet (uv) light. Generally, a small amount of an initiator molecule is used in the photopolymerization to absorb uv light and generate free radicals which polymerize the surfactants. Non-photopolymerization reactions using, for example, thermal initiators and/or other spontaneously decaying initiators can also be used to polymerize LLC assemblies. For thermal initiated systems, the initiator molecule must have a suitable half-life decay at a temperature that is below the clearing point for the LLC phase or aggregate.
Srisiri et al.,1997 report a method to produce phospholipid dienoyl molecules as polymerizable surfactants. Srisiri et al. report that the reactive diene group could be located on either or both lipid tails near the lipid backbone, i.e. the glycerol unit in the case of phospholipids.
Pindzola, et al., 2001 report that surfactants with a terminal diene can be polymerized and maintain their liquid crystal structure. Optionally, a divinylbenzene could be used to crosslink the diene monomers.
O'Brien et al., 1998 is a review article on polymerization of preformed self-organized assemblies. Examples of polymerizable lipids are provided. Polymerization of nonlamellar phases including the HII hexagonal phase and several bicontinuous cubic phases the structures of which are illustrated. The authors note that polymerization of bicontinuous cubic phases (bicontinuous means that the phase contains continuous polar (hydrophilic) and nonpolar (hydrophobic) regions) should yield materials with interpenetrating water channels (polar regions). FIG. 7 of the reference illustrates exemplary structures of such channel networks.
U.S. Pat. No. 5,849,215 (Gin et al.) and Gin et al., Acc. Chem. Res., 2001 report a method for synthesizing composites with architectural control on the nanometer scale. The patent reports an ordered nanocomposite of a matrix component and a filler component. The matrix component comprises polymerized inverse hexagonal phase-forming lyotropic liquid-crystalline (LLC) monomers and defines a hexagonally-packed array of tubular channels. The filler component, which is contained in the tubular channels of the material, is a solid or semi-solid component which provides structural integrity or adds other beneficial properties. Filer components are hydrophilic and compatible with the aqueous phase in the tubular channels. The polymerizable surfactant monomer system is used as an organic template, providing the underlying matrix and order of the composite system. Polymerization of the template in the presence of an optional crosslinking agent with retention of the liquid-crystalline order was followed by a second polymerization of a hydrophilic polymer precursor within the tubular channels of the polymer template.
The present invention relates to adding hydrophobic polymer to the organic (hydrophobic) domains of the LLC phase for the purpose of imparting physical of chemical properties to the nanocomposite such as forming an elastomeric material or an organic matrix or high chemical resistance. Subsequently, the surfactant molecules react with themselves to form covalent bonds.
While nanocomposites have been prepared employing polymerizable LLCs there remains a need in the art for composite materials with varying properties.